During medical imaging of the heart recordings have to be taken of a moving object, the beating heart. If the movement is not taken into account, the images are blurred. This problem occurs for example with imaging systems using computed tomography, magnetic resonance tomography, positron emission tomography and with ultrasound systems. With CT recordings in particular it is disadvantageous for recordings to have to be repeated, as the patient is then exposed to a higher radiation dose.
It is the convention to attempt to synchronize or trigger recordings of the heart in a prospective manner using the heart's electrical signal, in other words generally by means of an electrocardiogram (EKG). To this end electrodes are positioned on the skin of the patient and one or more derivations of the EKG are segmented for characteristic features, generally the so-called R-wave. The derivations are voltage measurements between two points of the body. The R-wave (see also FIG. 4) is the most characteristic feature of an EKG. It is the first positive deflection of the QRS complex. The QRS complex correlates with ventricular excitation or the depolarization of the two ventricles.
The actual scan or image recording then takes place in a defined interval relative to the position of the R-wave, so that the influence of the inherent movement of the beating heart can be reduced. The reliability of synchronization or triggering, in other words both the sensitivity and the predicativity, or the amount of information provided by the signal and its temporal constancy, in other words for example the deviation of the relative positions of the measured signals from the true R-wave, are decisive for the quality of the imaging here. A corresponding method is described in DE 10 2010 041 777 A1.
It is however disadvantageous that the electrical excitation of the heart does not correlate fully with the relevant variable, the mechanical excitation of the heart. There is a patient-specific offset between the R-wave and the ejection of the atria of the heart, which is also subject to dynamic fluctuations due to mechanical influences such as respiration or patient movement and autoregulatory processes.
There are also alternative approaches, for example the use of the pulse wave at the finger (PPG=photoplethysmography). However this has a time offset, which only allows triggering retrospectively and not in real time. The technology cannot therefore be used in particular with computed tomography due to the particularities cited above.
Alternatively it is also possible to use capacitive sensors, a ballistocardiogram, radar or phonocardiography, which allow the mechanical excitation of the heart to be shown to some extent. However the recorded signals generally do not have unique maxima but rather a distribution to different maxima. With a real-time recording it is initially unclear which of the maxima corresponds to the heart movement to be detected. It is therefore not easy to determine the movement time point of the heart reliably in real time and respond for example to an arrhythmia in the signal.
The maxima of the graphs showing the mechanical excitation of the heart are frequently not very marked so they can barely be distinguished from one another. Also the maxima often follow one another very closely in said graphs so their resolution may be very problematic. The comparison of the maxima with reference curves, for example as part of a filter procedure (matched filter), also encounters major problems, as the shape of the curves is only fully known as it were in retrospect but the movement of the heart has to be determined in a prospective manner in order to terminate imaging accordingly.
The direct measurement of a physical variable characterizing the mechanical excitation of the heart therefore also gives rise to the problem of being able to guarantee a reliable prediction of heart movement in real time. In this instance it is also difficult to respond to particularities such as extrasystoles, which are generally associated with a change in the muscle contraction pattern and can therefore be reflected very differently in the biosignals. However as the heart moves in such an instance, such events also have to be registered reliably.